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Flexural Capacity of Fire-Damaged Prestressed Concrete Box Beams

CHAPTER 5. Discussion of Results

The objective of this project was to determine if the incident on the Route 7 bridge had significantly degraded the flexural capacity of the adjacent box beams. The full scale flexural testing of four of these beams indicates that the live load elastic flexural capacity is approximately 515 kN-m (380 kip-ft). The same tests indicated that the live load ultimate flexural capacity is at least 1,206 kN-m (890 kip-ft). In terms of total load, the experimentally determined ultimate flexural capacity of these beams was determined to be at least 1,572 kN-m (1,160 kip-ft). For comparison, the analytically determined ultimate flexural capacity of these beams prior to the incident was 1,407 kN-m (1,038 kip-ft).

Although the four beams were originally located in different parts of the bridge, the large-scale test results and the visual examination of the bridge indicate that all the beams likely underwent a similar level of degradation during the incident. Given that the weakest of the four beams carried an ultimate flexural capacity that was more than 10 percent greater than the pre-incident analytically determined ultimate flexural capacity, it seems that the beams were not significantly structurally degraded by the fire.

However, the incident may have caused sufficient damage to the tensile-region concrete in these beams to impair their long-term flexural behavior. The as-built concrete cover below the midspan cross-section bottom row of strands varies between 33 and 46 mm (1.3 and 1.8 inches). Recall that the fire caused surface spalling on the bottom flange of the beams in many areas to a depth of 10 mm (0.4 inch), and the petrographic examination indicated that an additional 25 mm (1 inch) of intact concrete exhibits penetrating cracks caused by the fire. Combined, these facts seem to indicate that the bottom row of strands may have had decreased long-term protection from corrosive forces.